Teleseismic Relocation of Earthquakes along the Sumatra-Andaman Subduction Zone

The Sumatra-Andaman subduction zone presents a significant seismic hazard to Southeast Asia, as exemplified by the 2004, 2005, and 2007 great earthquakes occurring offshore Sumatra. However, investigations of seismicity associated with these events are limited by the large uncertainties associated with teleseismic location catalogs and by the paucity of more precise local seismic data in the region. In this study, we used a teleseismic double-difference method with three-dimensional ray tracing conducted through nested regional-global models to relocate teleseismicity from the Sumatra-Andaman region before and after the great earthquakes of 2004, 2005, and 2007. We tested the algorithm’s accuracy using independent local data and an alternate grid search relocation method and found good agreement between the results. The use of depth phases, cross-correlation and catalog differential times, and a realistic 3-D velocity model improves the accuracy and precision of epicenters and focal depths, systematically shifting them NE and shallower, respectively. The resulting relative location uncertainties are reduced to ~2 km, comparable to results from other teleseismic double-difference relocation studies. The relocations refine the location of the megathrust and other faults, the patterns of aftershocks, and their relation to slip during the 2004 and 2005 great earthquakes. In addition, the relocations reveal several discrete features not previously discernible in the scatter of teleseismic catalogs including a curvilinear feature at the updip limit of the 2005 rupture zone. Finally, relocations delineate the downdip continuation of the Investigator Fracture Zone and reveal an apparent slab tear associated with this feature, which may limit lateral rupture propagation in the region.